Electrolytic capacitors with pressure-relief and metal electrode structure retaining means

ABSTRACT

The disclosed electrolytic capacitors have an end-seal and a terminal at an end of a case containing an electrolytic capacitor section. A metal-disc barrier between the electrode assembly and the end-seal is retained in the case by a formation of the case for preventing expulsion of the electrode assembly upon occurrence of an internal pressure great enough to blow the end-seal out of its case-sealing position.

FIELD OF THE INVENTION

This invention relates to electrolytic capacitors, especially"miniature" capacitors having pressure-relief means.

BACKGROUND OF THE INVENTION

Over the years many pressure-relief structures have been incorporated inthe design of hermetically sealed electrolytic capacitors, often takingthe form of valves. The can is sealed to preserve the electrolyteagainst drying out or leaking. Explosive pressure can thus developinside the can due to malfunction or misapplication. Pressure-reliefstructures devised for large capacitors have been found impractical forincorporation into capacitors having small-size cans, e.g. about1/2-inch diameter and less. In the absence of a distinct pressure-reliefdevice, it has been known that the entire end-seal structure would bereleased suddenly upon development of internal pressures approachingexplosive levels and even at much lower pressures, representing a safetyhazard. The entire capacitor electrode assembly might then be projectedviolently and dangerously from the can.

As one solution to this problem for special application tosmall-diameter cans, it has been proposed that a weakening incision inthe end or the side wall of the can would provide safe pressure-relief.That approach, while effective, has required the exercise of great carein achieving precisely the right amount of penetration of the incisioninto the wall of the can. Despite care in making the incisions, thepressure level at which venting occurs is variable. If the incision istoo deep, the can vents too readily and the capacitor fails prematurely.If the incision is not deep enough, the pressure may well blow out theend seal before the incision functions. The incision cannot be tested tosort out the capacitors having the proper venting range since the testis destructive. Moreover, the incision represents a weak point thatcould develop into a leak. The incision can also be breachedaccidentally in handling, as by a fingernail, particularly where theincision is made on the outside surface of the can.

Another solution to the venting problem is disclosed and claimed in acompanion application of Fred H. West filed concurrently herewith. Thatapproach to the problem is based on accepting as non-hazardous therelease of the end closure of a capacitor in response to excessiveinternal pressure, particularly where the end seal is a resilient plug.It is shown there that a suitably rigid barrier inside the end-seal andtoo large to pass by an inward-projecting formation of the can, willeffectively prevent the electrode assembly from being expelled. Thepresently preferred embodiment of the broad concept in that applicationincorporates features of the present invention.

SUMMARY OF THE INVENTION

A metal barrier is interposed between the closure or end-seal and thewound-foil electrode section contained in the case of an electrolyticcapacitor. A formation of the case prevents the barrier from leaving thecase when internal pressure blows the end-seal free, and the barrier isfirm enough to block the electrode assembly from being expelledpowerfully when the end-seal is released by excessive internal pressure.The barrier is of a metal that is essentially inactive when exposed tothe electrolyte, and thus an aluminum barrier can safely be used in acapacitor having an aluminum can and aluminum electrodes. In capacitorshaving tantalum electrodes and a silver can, the barrier may be tantalumor silver. Further, to avoid accidental short-circuiting across theupper edges of the foil electrodes, the upper edge of the foil that isconnected to the riser is recessed in relation to the other foilelectrode. The metal disc has a hole through which the riser extends.The hole is over-size to avoid contact with the riser and to assureexposure of the end seal to internal pressures. The periphery of thebarrier is close to the wall of the case, thus centering the barrierabout the riser.

The nature of the invention, and its various objects and novel features,will be recognized and better appreciated from the following detaileddescription of an illustrative embodiment shown in the accompanyingdrawing.

THE DRAWINGS

In the drawings, the single FIGURE is a longitudinal cross-section of anovel electrolytic capacitor as an embodiment of the invention.

AN ILLUSTRATIVE EMBODIMENT

The illustrative capacitor in the drawing includes a generallycylindrical can 10 as of aluminum containing a wound electrolyticcapacitor section 12. The wound capacitor section here includesalternating convolutions of a cathode foil and an anode foil, separatedfrom each other by porous separators as of paper, the wound sectionbeing impregnated with an electrolyte. In this capacitor, one end of thecan is closed by end wall 10a that is an integral portion of can 10. Theside and end walls are formed of a single metal part, as a deep-drawncan. The opposite end of the can is hermetically sealed by a resilientplug 14 as of butyl rubber. Riser 16 of aluminum is connected to theanode foil and extends through plug 14 to anode terminal wire 18, wire18 being butt-welded to riser 16. The riser and the terminal wireconstitute the anode terminal. Cathode terminal wire 20 is butt-weldedto the exterior of end wall 10a of the can. The cathode foil 24 is widerthan the anode foil 26. One edge of the cathode foil extends from thelower end of the wound section and freely and firmly contacts can 10.Both edges of the anode foil are recessed from both top and bottom edgesof the cathode foil. A connecting strip of aluminum (not shown) extendsfrom the cathode foil to the can, welded to the foil and to the insideof the can in a conventional manner. The connecting strip is long andthe welds are done before the wound section is inserted.

Two formations 10b and 10c of the can squeeze resilient plug 14 in axialand radial directions and thereby form hermetic seals between the canand the plug and between riser 16 and plug 14. Bead 10c indents thelateral surface of plug 14. The lower or inner surface of plug 14 issupported by the assembly in the can. Pressure exerted by lip 10b on thetop or outer face of plug 14 is firmly resisted by bead 10c.

The illustrative capacitor as thus far described is a well-known type ofcapacitor. In so-called miniature capacitors, the can has a maximumdiameter (which varies with different capacitor ratings) of about1/2-inch. Its length varies widely, also depending on the ratings.

Riser 16 has a smooth cylindrical surface where it passes through plug14. Riser 16 has a diameter of 0.070 inch in an example. The outsidediameter of cylindrical can 10 in this example is nominally 0.375 inch,and its wall thickness is about 0.020 inch. The can is of deep-drawnaluminum. Such capacitors are made by assembling the capacitor sectioninto the can, slipping centrally pierced plug 14 into place along wire18 and riser 16, and sealing the capacitor by rolling-over lip 10b andforming bead 10c in spinning operations. In this example, bead 10c has aminimum external diameter of about 0.312 inch. Accordingly, the internaldiameter of bead 10c is 0.272 inch as compared with the internal candiameter of about 0.335 inch. Plug 14 in this example initially has adiameter of 0.320 inch and a thickness of 0.160 inch.

The capacitor section is impregnated with an electrolyte, either beforethe section is inserted into the can or after insertion but before theplug is sealed in place. The anode foil is treated to provide aso-called formed oxide, in a preparatory operation. The completedcapacitor is electrically treated to reform any damaged oxide and toform anodically all other bare areas of the anode that are exposed tothe electrolyte. The electrolyte, a liquid or a gel or the like,ordinarily is a formulation containing some water and in any case ittends to boil or to evolve gas when the capacitor malfunctions and whenit is misapplied. Hydrogen is commonly evolved, or both hydrogen andoxygen may be evolved in case of decomposition of water in theelectrolyte. The evolution of gas is rapid when the capacitor fails inuse, because the absorbed electrical energy tends to cause rapiddecomposition and/or boiling of the electrolyte. Evolving gas canincrease the internal pressure to the point where plug 14 or othercomparable end seal in prior-art forms of electrolytic capacitors isblown free. Where oxygen and hydrogen are both developed, there may evenbe an internal explosion caused by an internal spark that ignites themixture of gases yielded by electrolytically decomposed electrolyte.

Blowing of the capacitor's end seal or other form of pressure-reliefdevice must occur under these conditions. However, damage could resultfrom violent expulsion of capacitor section 12. For this reason, someform of pressure-relief or venting has been built into the containers ofelectrolytic capacitors for many years. The illustrated and presentlypreferred capacitor meets the problem directly and effectively byincluding a metal barrier 22 inside plug 14 and having a diameterdefinitely too large to pass formation 10c of the can. In this example,this is the same formation that squeezes plug 14 to seal the can to theplug and to seal the plug to the riser. Barrier 22 may be a disc ofaluminum having an outside diameter of 0.315 inch and a thickness of0.060 inch, and a hole of 0.150 inch diameter through which riser 16extends. The clearance between riser 16 and disc 22 provides space forinsulation where the disc is of metal and touches--or can touch--the canor the cathode foil or both. The projecting margins of the innerconvolutions of the cathode foil may extend into this clearance space;but suitable material such as the paper separators provides insulationin the space between the riser and the nearby portions of the cathodefoil. In any case, the enlarged hole provides a means for unrestrictedexposure of plug 14 to the gas pressure developed at the capacitorsection 12. Thus disc 22 does not seal plug 14 from the space containingthe electrolyte and consequently the end-seal, especially the plug typeof end-seal, is subjected to any internal pressure that may develop atthe section.

The upper edge of the anode foil 26 is recessed in relation to the upperedge of the cathode foil 24. This safeguards the assembly against apossibility of the anode foil becoming short-circuited to the cathodevia the metal disc. When end pressure is applied to the assembled partspreparatory to sealing plug 14 in place, even if the paper separatorsproject above the edges of both foils, short-circuiting might occur ifthe upper foil edges were approximately aligned. The foil edges couldpenetrate projecting paper separators and become short-circuited by themetal disc.

In the example given above, plug 14 is squeezed and severely deformedand retained in hermetic sealing contact with the can and the riser. Theplug blows open and relieves internal pressure when the pressure risesto about 500 pounds per square inch or somewhat less, in the case of candiameters of about 1/2-inch or less. This is far below the rupturingstrength of the can. The same pressure-relief occurs where disc 22 ispresent. However, disc 22 serves as a secure barrier against violentexpulsion of wound section 12. Riser 16 and wire 18 are united securelyto the anode foil, thus being a secure part of the electrode assembly.The riser and wire are not united to plug 14 and are free of obstructionthat could block plug 14 from outward pressure-relieving shift.Otherwise such an obstruction could cause plug 14 to pull on the woundsection and thus threaten forcible expulsion of the wound section. Ofcourse, a limited enlargement of terminal 16, 18 immediately outsideplug 14 can be present if it does not prevent pressure-responsivedisplacement of plug 14 apart from section 12. Where there is a readilyrupturable conductor between the wound section and the anode terminal,the terminal could be united to the end-seal without interfering withsafe pressure relief.

Barrier 22 in the above example has proved eminently successful where itwas shaped as a centrally perforated disc. However, the barrier can haveother shapes in capacitors having the illustrated form of end seal. Atthe loss of some strength, edge-notched discs or discs with plural holescan be used with the type of end plug illustrated. Where other forms ofend seals are used, barrier shapes should be chosen such as to avoidimpairment of the seal between the can and the end-seal, and between theriser and the end seal.

In the form of end seal illustrated, the plug is directly exposed, topand bottom, to lip 10b and to disc 22, respectively. Obviously, thinwafers could be interposed at these points without significant changesince those wafers would function as part of the plug in responding tointernal pressures.

In the described capacitor, disc 22 is cooperable with bead 10c inrestraining the capacitor section against expulsion, bead 10c in thisexample serving dual functions. It is evident, however, that a separateformation of the can could be included (a needless complication) forarresting barrier 22. Bead 10c would then be relied on only to serve itsseal-forming function.

Various other modifications will be obvious to those skilled in the art.Therefore, the invention should be construed broadly in accordance withits full spirit and scope.

We claim:
 1. An electrolytic capacitor, including a cylindrical metalcan, an electrode assembly in the can impregnated with electrolyte, aterminal extending outside the can from said electrode assembly and aresilient plug forming an end closure of the can, said can embodyingsealing means for squeezing the resilient plug and for thereby enforcingsealing contact of the plug to the can and to said terminal andincluding at least one formation of the can about the plug, said plugand said sealing means being proportioned to release the plug bodilyfrom its position as an end closure in response to internal pressureswell below case-rupturing pressures, and rigid metal barrier meansseparate from said plug interposed between said electrode assembly andplug and cooperable with an inward-projecting portion of the cylindricalwall of the can for preventing expulsion of said electrode assembly fromthe can upon release of said plug, said capacitor incorporating apassage bypassing said barrier means for transmitting gas pressure fromthe electrode assembly to the internal end of the plug.
 2. Anelectrolytic capacitor in accordance with claim 1, wherein said sealingmeans includes an annular bead projecting inward of the wall of thecase, said bead pressing into the lateral surface of the plug.
 3. Anelectrolytic capacitor in accordance with claim 1, wherein said sealingmeans embodied in the can includes a turned-in lip over an annular areaat the end of the plug and an inward-extending bead bearing against andsurrounding the lateral surface of the plug.
 4. An electrolyticcapacitor in accordance with claim 1, wherein said barrier has anoversized hole through which said terminal extends and which providesfor gas pressure transmission as aforesaid.
 5. An electrolytic capacitorin accordance with claim 1, wherein said sealing means for squeezing theplug constitutes the portion of the can with which said barrier means iscooperable for preventing expulsion of the electrode assembly.
 6. Anelectrolytic capacitor, including a case, an electrode assemblyimpregnated with electrolyte contained in the case, the case having anopening through which the electrode assembly could be expelled in caseof excessive pressure build-up in the case, an end-seal closing saidopening, and a terminal extending from an electrode of said assembly tothe exterior through the end-seal, the end-seal being displaceable fromits said case-closing position by excessive internal pressure, said casehaving an inward projecting obstruction, and a stable metal barrierwithin the case, separate from and inward of said end-seal andcooperable with said obstruction for barring expulsion of the electrodeassembly when the end-seal is displaced.
 7. An electrolytic capacitorassembly in accordance with claim 6, wherein the shape of said barrierin relation to the rest of the capacitor affords a gas pressuretransmission passage from the interior of the case to the inner surfaceof the end-seal by-passing the barrier.
 8. An electrolytic capacitorassembly in accordance with claim 6, wherein said terminal is firmlyconnected to the electrode assembly and wherein the terminal and the endseal are free of interlocking means such as would enforce outward travelof the electrode assembly with the end-seal when the latter is expelledby excessive internal pressure.
 9. An electrolytic capacitor assembly inaccordance with claim 6, wherein said case and said electrodes and saidbarrier are of aluminum.
 10. An electrolytic capacitor assembly inaccordance with claim 6, wherein the electrodes of said electrodeassembly are wound foil electrodes having respective upper edgesopposite to said barrier and wherein the foil electrode connected tosaid terminal has its upper edge recessed in relation to the upper edgeof the other foil electrode.